Field
This disclosure relates to archival data storage.
Description of the Related Art
Color photographic recording film is unsuitable for long-term archival storages of motion pictures and other imagery. Color film contains photographically active chemicals which remain in the film after development. These chemicals will continue to undergo chemical reactions which cumulatively cause color fading in the images. To avoid color fading, color motion picture imagery is commonly archived onto black and white film by dividing the color image into constituent red, green, and blue color channels. Each of these color channels is recorded onto a separate roll of black and white film. Each roll of film corresponding to a single color is known as a “record” with the totality of film rolls being known as a “separation master”. The developed black and white film is relatively stable (compared to color film) but still subject to degradation over long storage periods.
Conventional magnetic recording media are typically produced using a continuous wet coating process, where a magnetic dispersion is applied to the base film. This magnetic dispersion consists of binders, magnetic pigments, dispersants and lubricants which are dissolved in organic solvents to form a slurry. These binders (along with residual solvent, dispersants, etc.) will, over time, undergo chemical processes which may weaken their ability to hold the magnetic pigments to the base media or may cause breakdown of the pigments themselves due to chemical reactions. Thus magnetic storage media may also be unsuitable for archival storage of images and other data.
Digital optical tape system (DOTS) media based on a very stable, very sensitive phase-change recording material has a potential for archival storage of imagery, data, and documents for periods in excess of 100 years. It has been shown that DOTS media is stable for 100 years at 38° C. and over 200 years at room temperature (25° C.). Further, DOTS media is immune to chemical, water, and other environmental damage and is impervious to corrosion.
As described in U.S. Pat. No. 6,505,330, DOTS media may be formed by coating an alloy of Antimony, Tin, and, optionally, one or more additional metals onto a polymer film. The recording material may be coated on the film using a physical vapor deposition (PVD) process such as, for example, DC magnetron sputtering. The film may be a dimensionally stable polymer material such as, for example, polyethylene terephthalate (PET), commercially known as Mylar® or Estar®. The recording material experiences a phase change when heated to a temperature about 180° Centigrade. The phase change is accompanied by a change in optical reflectivity. The DOTs media has two states (i.e. any spot on the media either has or has not been heated to cause the phase change). Thus all information to be recorded on the DOTS media must be converted into binary data.
Data may be written onto the DOTS media by localized heating using a laser or other energy source. For example, U.S. Pat. No. 7,248,278 describes a printing system that may be suitable for writing data onto the DOTS media. A linear spatial light modulator is illuminated by an expanded laser beam, and an image of the spatial light modulator is projected onto a photosensitive surface, which could be the DOTS media.
Recorded data may be read from the DOTS media by detecting the localized optical reflectivity of the media. For example, U.S. Pat. No. 5,321,683 describes a system for reading the DOTS media. A line of data on the DOTS media is illuminated by a linearly expanded laser beam, and an image of the DOTS media is projected onto a detector array.
In this patent, a “color image” is an image with more than one color or hue, as opposed to a “black and white” image that is comprised of black, white, and intermediate shades of gray. A “film image” is an image recorded on photographic film, as opposed to and image captured or created digitally. A “film image” may be a positive or negative transparency or a print. A “digital image” is not literally an image that can be seen, but rather a set of digital data that represents an image and can be reconstructed into a visible image by, for example, by printing the data or presenting the data on a display.
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number where the element is introduced and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.